This is what the GNU website says about plugins. In short, it would most likely have to be licensed under the same one as Notepad++. However, keep in mind that you can still sell software that is licensed under GPL, you would also have to provide the source code as well. I’d advise thoroughly reading the entire GPL license.

To finalize this topic, the cases of Plugins seems to be “borderline” on the GPL licences restrictions. It seems that the way communication are done between plugin and npp doesn’t involve any restrictions for licencins for plugins.

If the program dynamically links plug-ins, and they make function calls to each other and share data structures, we believe they form a single program, which must be treated as an extension of both the main program and the plug-ins.

This is exactly how plugins are used from Notepad++. So I really don’t see a way that you wouldn’t have to use a GPL license. Again…“not a lawyer”.

Let me clarify a bit. I’ve seen a lot of cases where people have this question about programs licensed under the GPL and plugins. So I’ve always just referred to that particular section of the website. So if you’ve found some other resources or know other information I’d really be interested.